The invention relates to an optical assembly with an optical axis, comprising a holder with an interior space bounded by an inner wall. An optical element mounted in the holder has an outer wall, an adhesive being applied between the inner wall of the holder and the outer wall of the optical element.
Such optical assemblies are known from EP No. 0090218 (herewith incorporated by reference). The known assemblies each comprise an optical component, such as a round mirror or a lens, and a holder which forms part of an optical apparatus. The holder has a cylindrical bore in which the optical component is mounted, a cylindrical gap being formed between the inner wall of the holder and the outer wall of the optical component in the bore. This gap is wholly or partly filled with an adhesive agent such as a glue.
Optical assemblies are employed in optical apparatus, such as optical disc players, where it is important that the various optical elements are correctly positioned relative to each other. For the manufacturing process it is therefore essential that the optical elements in the optical assemblies are situated at the correct predetermined locations and in the correct position relative to the holder. Important in this respect is the positional accuracy in the direction of the optical axis (Z-direction) of the optical assembly, in the direction of two axes which extend perpendicularly to the optical axis and to each other (X-direction and Y-direction), and in the directions of rotation about the last-mentioned axes.
In one of the assemblies disclosed in EP 0090218 the adhesive is applied in the form of an annular adhesive layer, the layer also adhering to a wall portion of the holder which extends in a plane which is oriented transversely of the optical axis. However, this construction has the disadvantage that as it cures shrinkage of the applied adhesive is inevitable and gives rise to impermissible displacements of the optical component in the Z-direction. Moreover, the likelihood of the optical component being tilted about the X and Y-axes as a result of shrinkage stresses in the adhesive layer is substantial. The above positioning problem can be mitigated to some extent by reducing the thickness of the adhesive layer in the Z-direction but the required strength and stability of the adhesive bond distinctly limit this possibility. A suitable choice of the adhesive may also reduce said problem, but completely eliminating shrinkage and shrinkage stresses is found to be impossible. In another optical assembly disclosed in EP No. 0090218 the adhesive is applied in an annular space between the flat inner wall and outer wall. In yet another assembly the outer wall of the optical component is formed with a circumferential groove, the adhesive extending into this groove. These two assemblies both present the problem that the adhesive, which is applied in the form of a liquid or paste, will flow down after application in said space. This may give rise to at least a local formation of a bead of adhesive on the underside of the optical component, which may lead to positioning errors after curing of the adhesive. Moreover, the behaviour of the adhesive in the fairly large gap between the holder and the optical component is not predictable because generally the gap is not rotationally symmetrical as a result of the positioning of the optical component by means of a tool provided for this purpose, so that circumferentially varying shrinkage stresses may arise in the adhesive causing displacements, in particular tilting, of the optical components relative to the holder.